The present invention relates to an electric brake system, and more particularly to an electric brake system which has a rotational motion converting mechanism for converting rotational motion of a motor to linear motion.
Electromechanical brake-by-wire systems (EMB) have been developed in recent years. Japanese Patent Provisional Publication No. 8-284980 discloses a typical EMB which employs a ball screw mechanism for converting rotational motion of a motor into linear motion. The EMB is arranged to linearly move a brake pad by means of the linear motion of the ball screw mechanism and to control braking force through the control of the motion of the brake pad relative to a disc rotor. The ball screw mechanism performs small friction during the conversion from the rotational motion of the motor into the linear motion.
However, it is difficult to sufficiently lubricate a screw-shaft ordinary use region, which contacts with balls of the ball screw during an ordinary braking operation of the brake system, in order to maintain the small friction performance.
It is therefore an object of the present invention to provide an improved electromechanical brake system which enables oil films to be sufficiently provide oil films on the screw shaft and ensures smooth braking operation.
An electromechanical brake system according to the present invention is for a vehicle and comprises a brake actuator installed to each wheel of the vehicle, a brake pedal sensor for detecting a depression quantity of a brake pedal and a controller connected to the brake actuator and the brake pedal sensor. The brake actuator comprises a motor, a disc rotor integrally connected to each wheel of the vehicle, a pair of brake pads for clamping the disc rotor according to an operation of the motor, and a rotational motion converting mechanism for converting rotational motion of the motor into linear motion to move the brake pad relative to the disc rotor. The controller drives the motor of the brake actuator within a braking operation range so as to generate a braking force according the depression quantity of the brake pedal and drives the motor over the brake operation range to execute a lubricating operation of the rotational motion converting mechanism when the vehicle is put in a stopping state.